Bipolar junction transistor (BJT) base conductor pullback

ABSTRACT

Some embodiments are directed to a bipolar junction transistor (BJT) with a collector region formed within a body of a semiconductor substrate, and an emitter region arranged over an upper surface of the semiconductor substrate. The BJT includes a base region arranged over the upper surface of the semiconductor substrate, which vertically separates the emitter and collector regions. The base region is arranged within, and in contact with, a conductive base layer, which delivers current to the base region. The base region includes a planar bottom surface, which increases contact area between the base region and the semiconductor substrate, thus decreasing resistance at the collector/base junction, over some conventional approaches. The base region can also include substantially vertical sidewalls, which increases contact area between the base region and the conductive base layer, thus improving current delivery to the base region.

BACKGROUND

Bipolar junction transistors (BJTs) are commonly used in digital andanalog integrated circuit (IC) devices for high frequency applications.A BJT includes two p-n junctions sharing a cathode or anode region,which is called the base. The base separates two regions having a sameconductivity type, called the emitter and the collector, which isopposite of the conductivity type of the base. Depending on theconductivity types, a BJT can be of the NPN variety or the PNP variety.

A heterojunction bipolar transistor (HBT) is a type of BJT that usesdifferent semiconductor materials for the emitter/collector and thebase. By using different materials, the HBT reduces injection of holesfrom the base into the emitter region over the BJT. Consequently, theHBT supports higher frequencies than a BJT (e.g., several hundred GHz).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A-1B illustrate cross-sectional views of some embodiments ofbipolar junction transistors (BJTs) in accordance with the presentdisclosure.

FIG. 2 illustrates cross-sectional views of some embodiments of BJT inaccordance with the present disclosure.

FIG. 3 illustrates a flow chart of some embodiments of a method offorming a BJT in accordance with the present disclosure.

FIGS. 4-19 illustrate some embodiments of a series of cross-sectionalviews that collectively depict formation of BJT in accordance with thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Moreover, “first”, “second”, “third”, etc. may be used herein for easeof description to distinguish between different elements of a figure ora series of figures. “First”, “second”, “third”, etc. are not intendedto be descriptive of the corresponding element. Therefore, “a firstlayer” described in connection with a first figure may not necessarilycorresponding to a “first layer” described in connection with anotherfigure.

In some approaches, a vertical BJT is manufactured by forming acollector region (e.g., n-type region) in a semiconductor substrate,forming a base dielectric layer over the collector region, and forming apolysilicon base layer over the base dielectric layer. An etch is thencarried out to form an opening which extends through the polysiliconbase layer and which terminates at an uppermost surface of the basedielectric layer. A protective liner is formed in the opening to coversidewalls of the polysilicon base layer and terminate on the uppermostsurface of the base dielectric layer. Subsequently, while the protectiveliner sidewalls are left in place, a bottom portion of the protectiveliner is removed to expose the uppermost surface of the base dielectriclayer. An etch is then performed on this exposed surface to form arecess in the base dielectric layer. This recess extends verticallydownwards to an upper portion of the collector region and laterallyundercuts the protective sidewalls and edges of the base polysiliconlayer nearest the protective sidewalls. A base region (e.g., p-typeregion) is then selectively grown in the recess and meets the collectorregion at a collector/base junction, and an emitter region (e.g., n-typeregion) is subsequently formed over the base region and meets the baseregion at a base/emitter junction. Thus, a vertical BJT is established.Unfortunately, the recess in which the base region is selectively grownonly abuts the bottom planar surface of the base polysilicon layer, andthus, when the base is selectively grown in this recess, there is arelatively small contact area (i.e., high contact resistance) betweenthe base region and base polysilicon layer. Further, the recess oftenhas a rounded (e.g., concave) bottom surface, due to the etchingconditions employed. This rounded surface can limit the area of thecollector/base junction somewhat, limiting the gain of the BJT.

Therefore, the present disclosure is directed to a vertical BJT wherethe recess in which the base region is grown extends under the basepolysilicon layer and also extends laterally into the base polysiliconlayer as well. Thus, when the base region is selectively grown, inaddition to contacting an underside of the conductive base layer, thebase region contacts sidewalls of the conductive base layer to increasethe contact area (i.e., decrease contact resistance) compared to otherapproaches. In addition, rather than a rounded bottom surface, in someembodiments the base region can include a planar bottom surface, whichincreases contact area between the base region and the collector regionand correspondingly decreases resistance at the collector/base junction,compared to other approaches.

FIG. 1A illustrates a cross-sectional view of some embodiments of a BJT100. The BJT 100 includes a collector region 104 having a firstconductivity type (e.g., n-type with a first doping concentration),which is arranged within a semiconductor substrate 102. A base region118 (e.g., p-type) is arranged over the collector region 104, and meetsthe collector region 104 at a collector/base junction 122. An emitterregion 106 having the first conductivity type (e.g., n-type with asecond doping concentration that is greater than the first dopingconcentration) is arranged over the base region 118, and meets the baseregion 118 at a base/emitter junction 120. The collector region 104,base region 118, and emitter region 106 are each made of semiconductormaterial, and are often in an n-p-n arrangement as this provides forhigher performance, although p-n-p arrangements are also possible. Insome embodiments of the BJT 100, the substrate 102 is silicon, the baseregion 118 is silicon-germanium, and the emitter region 106 ispolysilicon. When silicon-germanium or other semiconductor materialshaving a bandgap that is narrower than that of silicon is used for thebase region 118, the resultant bipolar BJT tends to have a higher gaincompared to BJTs where silicon is used for the base region, but bothapproaches are contemplated as falling within this disclosure.

FIG. 1B illustrates an exploded cross-sectional view of the BJT 100. Asshown in FIGS. 1A-1B, a base dielectric layer 112 separates a conductivebase layer 110 from an upper substrate surface 108. In some embodiments,the conductive base layer 110 is polysilicon and has the firstconductivity type, and the base dielectric layer 112 is an oxide (e.g.,SiO₂) which is less than 500 angstroms (Å) thick. A spacer layer 116(e.g., silicon nitride (SiN)) is arranged along vertical sidewalls 130of the emitter region 106, and electrically isolates a lower portion ofthe emitter region 106 from the conductive base layer 110. An inter-polydielectric (IPD) layer 115 (e.g., SiN) separates outer edges of an upperportion of the emitter region 106 from the conductive base layer 110.

For the embodiments of FIGS. 1A-1B, the emitter region 106 and thecollector region 104 include n-type silicon (Si) and the base region 118is p-type SiGe. Consequently, for the embodiments of FIGS. 1A-1B, theemitter region 106, the base region 118, and the collector region 104form an n-p-n junction of a heterojunction bipolar transistor (HBT).Charge flow through the BJT 100 results from diffusion of chargecarriers across the emitter/base junction 120, through the base region118, and across the collector/base junction 122. The charge flow resultsfrom independent biasing of the emitter region 106, the base region 118,and the collector region 104 by first through third contacts 128A-128C.In various embodiments, the first through third contacts 128A-128Cinclude one or more conductive materials including copper (Cu), aluminum(Al), tungsten (W), etc. The first contact 128A connects to the emitterregion 106, the second contact 128B connects to the base region 118through the conductive base layer 110, and the third contact 128Cconnects to the collection region 104. The first through third contacts128A-128C are arranged within an inter-layer dielectric (ILD) 140.

In an “on” state of the BJT 100, the first through third contacts128A-128C are biased such that there is a positive potential differencebetween the base region 118 and both the emitter region 106 and thecollector region 104 (i.e., positive potential difference across theemitter/base junction 120 and the collector/base junction 122). As aresult, electrons are injected from the emitter region 106 into the baseregion 118. The electrons are minority carriers within the p-type baseregion 118, which diffuse toward the collector region 104. Most of thecurrent though the BJT 100 is carried by the electrons moving throughthe p-type base region 118. A small portion of the current may alsoresult from a recombination of charge carriers (i.e., electrons andholes) at the emitter/base junction 120 and the collector/base junction122.

Returning to FIG. 1B, the BJT 100 has an advantage over someconventional BJTs in that is has an increased contact area between theconductive base layer 110 and the base region 118, along verticalsidewalls 132 and an upper surface 134 of the base region 118. Inparticular, the vertical sidewalls 132 extend continuously from theupper surface 134 of the base region 118 to the upper surface 108 of thesemiconductor substrate 102. In some conventional approaches, the baseregion 118 only contacts the conductive base layer 110 along its uppersurface 134, which decreases contact area and hence increases resistancerelative to the BJT 100.

The BJT 100 has a further advantage over some conventional BJTs ofincreased contact area between the base region 118 and the collectorregion 104 of the semiconductor substrate 102 along a planar bottomsurface 136 of the base region 118. In the conventional BJT, the baseregion 118 has a rounded bottom surface, which consequently reducescontact area and thus increases resistance at the collector/basejunction 122 of the BJT 100.

Increasing the contact area between the conductive base layer 110 andthe base region 118, as well as along the collector/base junction 122,also increases a cut-off frequency of the BJT 100 due to transit timereductions across the collector/base junction 122. In addition, byforming the opening within the conductive base layer 110 instead of adielectric layer arranged below the conductive base layer 110, thedielectric layer can be eliminated. As a result, a step height 138between a top of the emitter region 106 and the upper surface 108 of thesemiconductor substrate 102 can be reduced over the some conventionalmethods.

It is also noted that BJT 100 includes first through third shallowtrench isolation (STI) structures 124A-124C (e.g., oxide) arrangedwithin the upper surface 108 of the semiconductor substrate 102. Thefirst and third STI structures 124A, 124C are laterally separated by thecollector region 104. The first and second STI structures 124A, 124B arelaterally separated by a region of the semiconductor substrate 102 (andcollector region 104) located below the SiGe base region 118. The BJT100 also includes first and second deep trench isolation (DTI)structures 126A, 126B, which laterally isolate the collector region 104from other regions of the semiconductor substrate 102.

FIG. 2 illustrates a cross-sectional view of a BJT 200 in accordancewith some embodiments. Whereas FIG. 1A's BJT 100 has an emitter region106 with a “T-shaped” cross section, FIG. 2's BJT 200 has a an emitterregion 206 with a “pi-shaped” cross section, which is essentially twoT-shaped emitter regions 106 merged together. The pi-shaped emitterregion 106 connects to first and second base regions 118A, 118B, whichconnect to a collector region 104 of a semiconductor substrate 102. Thepi-shaped emitter region 206 of the BJT 200 essentially doubles theelectron current over the T-shaped emitter region 106 of the BJT 100, bydoubling the cross-sectional area of the emitter/base and collector/basejunctions.

The BJT 200 includes first through fourth STI structures 224A-224D,which are arranged within an upper surface 108 of the semiconductorsubstrate 102. The first and fourth STI structures 224A, 224D arelaterally separated by the collector region 104. The second STIstructure 224B laterally separates portions of the collector region 104directly below the first and second base regions 118A, 118B. The BJT 200also includes first and second DTI structures 126A, 126B, whichlaterally isolate the collector region 104 from other regions of thesemiconductor substrate 102. The first and fourth STI structures 224A,224D extend from the upper surface 108 of the semiconductor substrate102 to upper portions of the first and second DTI structures 126A, 126B,respectively. The first and second DTI structures 126A, 126B extend fromthe first and fourth STI structures 224A, 224D to an upper portion of aburied oxide (BOX) isolation structure 202. Consequently, the first andfourth STI structures 224A, 224D, the first and second DTI structures126A, 126B, and the BOX isolation structure 202, surround the collectorregion 104 and electrically isolate it from other regions of thesemiconductor substrate 102.

FIG. 3 illustrates a flow chart of some embodiments of a method 300 offorming a BJT in accordance with the present disclosure. While themethod 300 is described as a series of acts or events, it will beappreciated that the illustrated ordering of such acts or events are notto be interpreted in a limiting sense. For example, some acts may occurin different orders and/or concurrently with other acts or events apartfrom those illustrated and/or described herein. In addition, not allillustrated acts may be required to implement one or more aspects orembodiments of the description herein. Further, one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

At 302, a collector region having a first conductivity type (e.g.,n-type or p-type) is formed within a body of a semiconductor substrate.In some embodiments, the collector region is formed by an implant of oneor more dopants into a body of the semiconductor substrate.

At 304, a base dielectric layer is formed over an upper surface of thesemiconductor substrate. In some embodiments, the base dielectric layerincludes an oxide is formed by chemical vapor deposition (CVD),oxidation of the upper surface of the semiconductor substrate, or otherappropriate dielectric layer formation technique.

At 306, a conductive base layer is formed over the base dielectriclayer. The conductive base layer has a second conductivity type that(e.g., p-type or n-type, respectively), which is different from thefirst conductivity type. In some embodiments, the conductive base layeris formed by CVD or other appropriate layer formation technique.

At 308, a recess is formed within the conductive base layer.

At 310, a dielectric spacer layer is formed along vertical sidewalls of,and overlying a bottom surface of, the recess. In some embodiments, thespacer layer is formed by CVD or other appropriate layer formationtechnique.

At 312, the portion of the spacer layer overlaying the bottom surface ofthe recess is removed through a first etch process to expose a portionof the conductive base layer. In some embodiments, the first etchprocess is an anisotropic dry etch that removes the portion of thespacer layer overlaying the bottom surface of the recess.

At 314, the portion of the conductive base layer underlying the bottomsurface of the recess is removed through a second etch process, whichexposes a portion of the base dielectric layer. In some embodiments, thesecond etch process is an isotropic selective dry etch, which etches therecess vertically to the upper surface of the base dielectric layer, andlaterally within the conductive base layer, while leaving the basedielectric layer substantially intact. The resulting recess has across-sectional profile of an inverted T-shape. The inverted T-shapedrecess includes a horizontal portion formed within the conductive baselayer. In some embodiments, the horizontal portion includes a planarbottom surface that abuts a top surface of the base dielectric layer;the horizontal portion can also include substantially planar verticalsidewalls. In other embodiments, the sidewalls can be rounded, due tonature of the isotropic etch.

At 316, the portion of the base dielectric layer underlying the bottomsurface of the recess is removed through a third etch process, whichexposes a portion of the upper surface of the semiconductor substrate.In some embodiments, the third etch process is a selective etch with anetch selectivity between the base dielectric layer and the conductivebase layer, such that it etches the portions of the base dielectriclayer underlying the bottom surface of the recess while leaving theconductive base layer substantially intact. Consequently, the recess hasa planar bottom surface that abuts the upper surface of thesemiconductor substrate.

At 318, SiGe is selectively disposed along the upper surface of thesemiconductor substrate, and along sidewalls of the conductive baselayer. The SiGe forms a base region coupled to the conductive baselayer, the base region having the second conductivity type.Consequently, the SiGe base region has a rectangular shape orsubstantially rectangular shape, with a flat/planar bottom surface thatcontacts the upper surface of the semiconductor substrate. In someembodiments, the SiGe base region also has planar or substantiallysidewalls, a portion of which contacts the conductive base layer, and aplanar or substantially planar upper surface, a portion of which alsocontacts the conductive base layer. The base region can also haverounded sidewalls and/or a rounded upper planar surface.

At 320, a semiconductor or conductive material having the firstconductivity type is formed within the recess over the SiGe base region,to form an emitter region of the BJT.

FIGS. 4-19 illustrate some embodiments of a series of cross-sectionalviews that collectively depict formation of a BJT in accordance with thepresent disclosure. Although FIGS. 4-19 are described in relation to themethod 300, it will be appreciated that the structures disclosed inFIGS. 4-19 are not limited to the method 300, but instead may standalone as structures independent of the method 300. Similarly, althoughthe method 300 is described in relation to FIGS. 4-19, it will beappreciated that the method 300 is not limited to the structuresdisclosed in FIGS. 4-19, but instead may stand alone independent of thestructures disclosed in FIGS. 4-19.

In FIG. 4, which corresponds to act 302 of the method 300, the collectorregion 104 having a first conductivity type has been formed within abody of a semiconductor substrate 102. The collector region 104 has beenformed through the implantation of one or more dopants 402 through anopening of a mask 406 (e.g., SiN), which has been disposed and patternedover an upper surface 108 of the semiconductor substrate 102. For theembodiments of FIGS. 4-19, the first conductivity type is n-type, andthe dopants 402 include donors such as phosphorous (P), arsenic (As),antimony (Sb), or bismuth (Bi), etc., which forms an n-type collectorregion 104. The substrate 102 can be a bulk silicon substrate, asilicon-on-insulator substrate, a binary compound semiconductorsubstrate, ternary compound semiconductor substrate, or higher ordercompound semiconductor substrate, among others.

In FIG. 5, first and second deep trenches 502A, 502B have been formedwithin the semiconductor substrate 102 through an etch. The etch uses amask 504, which has been disposed over an upper surface 108 of thesemiconductor substrate 102, and which has then been patterned to formfirst and second openings 506A, 506B, which correspond to the first andsecond deep trenches 502A, 502B. The semiconductor substrate 102 hasthen been exposed to an etchant 508 (e.g., wet or dry etchant) accordingto the mask 504 to form the first and second deep trenches 502A, 502Bhaving a first depth 510.

In FIG. 6, first through third shallow trenches 602A-602C have beenformed within the semiconductor substrate 102 through an etch. The etchuses a mask 604 that has been disposed over an upper surface 108 of thesemiconductor substrate 102. The mask 604 has then been patterned toform first through third openings 606A-606C, which correspond to thefirst through third shallow trenches 602A-602C. The upper surface 108 ofthe semiconductor substrate 102 has then been exposed to an etchant 608according to the mask 604 to form the first through third shallowtrenches 602A-602C. The first and third shallow trenches 602A, 602C arecentered over the first and second deep trenches 502A, 502B. The firstand second deep trenches 502A, 502B extend from lower surfaces 612A,612B of the first and third shallow trenches 602A, 602C to a seconddepth 610, which is less than the first depth 510.

In FIG. 7, the first through third shallow trenches 502A-502C and thefirst and second deep trenches 602A, 602B have been filled with adielectric material (e.g., SiO₂) to form first through third STIstructures 124A-124C and first and second DTI structures 126A, 126B. Invarious embodiments, filling with the dielectric material includesdeposition process(es) such as CVD (e.g., low-pressure CVD (LPCVD) orplasma-enhanced CVD (PECVD)), physical vapor deposition (PVD), atomiclayer deposition (ALD), molecular beam epitaxy (MBE), electron beam(e-beam) epitaxy, or other appropriate process. Upon formation of thefirst through third STI structures 124A-124C and first and second DTIstructures 126A, 126B, excess dielectric material is removed from theupper surface 108 of the semiconductor substrate 102 through aplanarization process such as a chemical mechanical polish (CMP).

In FIG. 8, which corresponds to acts 304 and 306 of the method 300, abase dielectric layer 112 has been formed over the upper surface 108 ofthe semiconductor substrate 102 by CVD, wet or dry oxidation, or otherappropriate process. A conductive base layer 110 (e.g., polysilicon) hasthen been formed over the base dielectric layer 112. An IPD layer 115(e.g., SiN) has then been formed over the conductive base layer 110.

In FIG. 9, which corresponds to act 308 of the method 300, a recess 902has been formed within the conductive base layer. In some embodiments,the recess 902 is formed through an etch. The etch uses a mask 904, withan opening 906 that corresponds to the recess 902. The semiconductorsubstrate 102 has then been exposed to an etchant 908 according to themask 904 to form the recess 902. The recess 902 extends downward to abottom surface 910 which is vertically within the conductive base layermaterial 110 (e.g., polysilicon). Thus, the recess 902 extends partiallyinto the conductive base layer 110, but not completely through theconductive base layer material 110. For example, in some embodiments,the recess 902 can extend to a depth of between 10% and 90% of the totalthickness of the conductive base layer 110, or to a depth of between 40%and 60% of the total thickness in various embodiments.

In FIG. 10, which corresponds to act 310 of the method 300, a spacerlayer 1002 has been formed along vertical sidewalls 1004 of the recess902. The spacer layer 1002 can be a conformal layer that overlays thebottom surface 910 of the recess 902, as well as an upper surface 1006of the IPD layer 115. In some embodiments, the spacer layer 1002 hasbeen formed by CVD or other appropriate layer formation technique.

In FIG. 11, which corresponds to act 312 of the method 300, a portion ofthe spacer layer 1002 overlaying the bottom surface 910 of the recess902 has been removed through a first etch process, such that the bottomsurface 910 of the recess 902 again corresponds to a portion of theconductive base layer material 110. In some embodiments, the first etchprocess is an anisotropic dry etch with one or more etchants 1108including CHF₃ and/or CF₄ to form dielectric spacer.

In FIG. 12, which corresponds to act 314 of the method 300, the portionof the conductive base layer 110 underlying the recess 902 has beenremoved through a second etch and a third etch process, which exposes aportion of the base dielectric layer 112. In some embodiments, thesecond etch process is an anistropic and selective dry etch by HBr andO₂ to stop on 112. The third etch is an isotropic etch with aselectivity between the spacer layer 1002/base dielectric layer 112 andthe conductive base layer 110, such that it etches the conductive baselayer 110 while leaving the spacer layer 1002 and the base dielectriclayer 112 substantially intact. The gas ratio of SF₆:O₂ is less than 1with zero bias power. Consequently, the third etch etches the recess 902vertically to an upper surface 1206 of the base dielectric layer 112.The third etch also etches the recess 902 laterally within theconductive base layer 110. The resulting recess 902 has across-sectional profile of an inverted T-shape. The inverted T-shapedrecess includes a horizontal portion 1202 formed within the conductivebase layer 110. The horizontal portion 1202 includes a planar bottomsurface 910 that abuts an upper surface 1206 of the base dielectriclayer 112. In some embodiments, the horizontal portion 1202 alsoincludes substantially vertical sidewalls 1204, but in other embodimentsthe sidewalls 1204 will be rounded. In some embodiments, the third etchprocess is an isotropic etch with one or more etchants 1208 includingsulfur hexafluoride (SF₆) and oxygen (O₂).

In FIG. 13, which corresponds to act 316 of the method 300, the portionof the base dielectric layer 112 underlying the recess 902 has beenremoved through a forth etch process, such that its bottom surface 910of the recess 902 is coincident with the upper surface 108 of thesemiconductor substrate 102. In some embodiments, the forth etch processincludes a selective etch with an etch selectivity between theconductive base layer 110/spacer layer 1002 and the base dielectriclayer 112, such that it etches the portions of the base dielectric layer112 underlying the bottom surface 910 of the recess 902 while leavingthe conductive base layer 110 and the spacer layer 1002 substantiallyintact. Consequently, the recess has a planar bottom surface that abutsthe upper surface of the dielectric layer. In some embodiments, theforth etch process utilizes wet etch 1308 including fluoride compoundssuch as hydrofluoric acid (HF). Although this forth etch process isillustrated as terminating at the upper substrate surface 108 in FIG.13, in other embodiments, the forth etch can extend slightly downwardinto the substrate 102 such that the lower surface of the recess 1202 isbelow the interface between the base dielectric layer 112 and the uppersubstrate surface 108.

In FIG. 14A, which corresponds to act 318 of the method 300, asemiconductor material 1418 having a second conductivity type (e.g.,p-type), which is opposite the first conductivity type, is selectivelydisposed along the bottom surface 910 of the recess 902 on the uppersurface 108 of the semiconductor substrate 102, and along sidewalls 1204of the conductive base layer 110. In some embodiments, the semiconductormaterial 1418 includes SiGe, and the semiconductor substrate 102 andconductive base layer 110 includes silicon (Si). Consequently, the SiGebonds epitaxially to the Si of the semiconductor substrate 102 andconductive base layer 110, but not to the base dielectric layer 112(e.g., SiO₂). FIG. 14B illustrates such an embodiment, where theselectively grown semiconductor material 1418 forms a SiGe base region118, having a rectangular shape with substantially vertical sidewalls132, and a planar bottom surface 136 along the collector/base junction122. Although not illustrated, the bottom of the base region 1418 canextend below upper substrate surface 108 in some embodiments, and/or thesidewalls 132 can be rounded.

FIG. 14C illustrates an additional embodiment, where the selectivelygrown semiconductor material 1418 includes facets 1402 formed along thevertical sidewalls 132. The facets 1402 result from a lack of epitaxialadhesion between the material 1418 and the base dielectric layer 112.The facets 1402 result in small gaps and non-planar portions of thevertical sidewalls 132 along the base dielectric layer 112.

In some embodiments, a concentration of germanium (Ge) in the SiGe baseregion 118 is graded, making a bandgap of the SiGe narrower at thecollector/base junction 122 than at the emitter/base junction 120. Thegrading of the Ge concentration leads to a field-assisted transport inof electrons within the SiGe base region 118, which creates anaccelerating electric field the electron diffusing through the SiGe baseregion 118. The grading of the Ge concentration consequently increaseselectron diffusion through the SiGe base region 118.

In FIG. 15, which corresponds to act 320 of the method 300, a remainingportion of the recess 902 has been filled with a semiconductor orconductive material 1502 having the first conductivity type. Thesemiconductor or conductive material 1502 extends over the upper surface1006 of the IPD layer 115. In some embodiments, the semiconductor orconductive material 1502 includes polysilicon, which has been doped withone or more donor dopants such as P, As, Sb, Bi, etc., and which hasbeen disposed by CVD or other appropriate layer disposal technique. Anupper surface 1504 of the semiconductor or conductive material 1502 hasthen been planarized by a CMP or other appropriate process.

In FIG. 16, which corresponds to act 320 of the method 300, the material1502 (and underlying IPD layer 115) has been etched to form an emitterregion 106. The etch uses a mask 1604 that has been disposed over theupper surface 108 of the semiconductor substrate 102, and patterned. Thematerial 1502 (and underlying IPD layer 115) has then been exposed to anetchant 1608 according to the mask 1604 to form the emitter region 106.In various embodiments, the etchant 1608 includes a wet or dry etchant.As a result of the etch, vertical sidewalls 1502 of the emitter 106 andthe IPD layer 115 form a continuous planar surface 1602, which extendsfrom an upper surface 1606 of the emitter region 106 to the uppersurface 1610 of the conductive base layer 110.

In FIG. 17, which corresponds to act 320 of the method 300, theconductive base layer 110 (and underlying base dielectric layer 112) hasbeen etched to expose a portion 1702 of the upper surface 108 of thesemiconductor substrate 102. The etch uses a mask 1704 that has beendisposed and patterned. The conductive base layer 110 (and underlyingbase dielectric layer 112) has then been exposed to an etchant 1708(e.g., a wet or dry etchant) according to the mask 1704.

In FIG. 18, which corresponds to act 320 of the method 300, an ILD 140has been disposed over the upper surface 108 of the semiconductorsubstrate 102. In various embodiments, ILD 140 includes an oxide orlow-k dielectric material.

In FIG. 19, which corresponds to act 320 of the method 300, firstthrough third contacts 128A-128C have been formed within the ILD 140. Insome embodiments, formation of the first through third contacts128A-128C includes an etch of the ILD 140 to form trenches correspondingto the first through third contacts 128A-128C. The trenches have thenbeen filled with a one or more conductive materials (e.g., Cu, Al, W,etc.) to form the first through third contacts 128A-128C.

Therefore, the present disclosure is directed to a bipolar junctiontransistor (BJT) with a collector region formed within a body of asemiconductor substrate, and an emitter region arranged over an uppersurface of the semiconductor substrate. The BJT includes a base regionarranged over the upper surface of the semiconductor substrate, whichvertically separates the emitter and collector regions. The base regionis arranged within, and in contact with, a conductive base layer, whichdelivers current to the base region. The base region includes a planarbottom surface, which increases contact area between the base region andthe semiconductor substrate, thus decreasing resistance at thecollector/base junction, over some conventional approaches. The baseregion also includes substantially vertical sidewalls, which increasescontact area between the base region and the conductive base layer, thusimproving current delivery to the base region.

Some embodiments relate to a transistor, comprising a collector regionarranged within a body of a semiconductor substrate, the collectorregion having a first conductivity type. The transistor also comprises abase dielectric arranged over an upper surface of the semiconductorsubstrate and including a base dielectric opening over the collectorregion. The transistor further comprises a SiGe base region arrangedwithin the base dielectric opening, the SiGe base region having a secondconductivity type that is different from the first conductivity type andmeeting the collector region at a collector/base junction. Thetransistor further comprises a polysilicon base layer contacting both anupper surface and sidewalls of the SiGe base region and extendinglaterally over an upper surface of the base dielectric. The transistorfurther comprises a polysilicon emitter region arranged over the SiGebase region. The emitter region has the first conductivity type andmeeting the SiGe base region at a base/emitter junction.

Other embodiments relate to a method, comprising forming a collectorregion within a body of a semiconductor substrate, the collector regionhaving a first conductivity type. The method also comprises forming adielectric layer over an upper surface of the semiconductor substrate,and forming a conductive base layer over the dielectric layer. Theconductive base layer has a second conductivity type, which is differentfrom the first conductivity type. The method also comprises forming arecess within the conductive base layer, and forming a spacer layeralong vertical sidewalls of, and overlying a bottom surface of, therecess. The method further comprises extending the recess through aportion of the spacer layer overlaying the bottom surface of the recessthrough a first etch process. The method further comprises extending therecess through a portion of the conductive base layer underlying thebottom surface of the recess through a second etch process, such thatthe bottom surface of the recess abuts an upper surface of thedielectric layer. The method further comprises extending the recessthrough a portion of the dielectric layer underlying the bottom surfaceof the recess through a third etch process, such that the bottom surfaceof the recess abuts the upper surface of the semiconductor substrate.The method further comprises selectively disposing silicon-germanium(SiGe) along the bottom surface of the recess on the upper surface ofthe semiconductor substrate, and along sidewalls of the conductive baselayer, wherein the SiGe forms a base region within the conductive baselayer, the base region having the second conductivity type.

Still other embodiments relate to a transistor, comprising a collectorregion formed within a body of a semiconductor substrate, the collectorregion having a first conductivity type. The transistor also comprisesan emitter region arranged over an upper surface of the semiconductorsubstrate, the emitter region having the first conductivity type. Thetransistor further comprises a silicon-germanium (SiGe) base regionarranged over the upper surface of the semiconductor substrate andvertically separating the emitter and collector regions, the SiGe baseregion having a second conductivity type that is different from thefirst conductivity type and having substantially vertical sidewalls,which extend continuously from an upper surface of the SiGe base regionto the upper surface of the semiconductor substrate.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method, comprising: forming a collector regionwithin a body of a semiconductor substrate, the collector region havinga first conductivity type; forming a dielectric layer over an uppersurface of the semiconductor substrate; forming a conductive base layerover the dielectric layer, the conductive base layer having a thicknessmeasured from its uppermost surface to its lowermost surface and havinga second conductivity type that is different from the first conductivitytype; forming a recess within the conductive base layer, the recessterminating at a first depth measured from the uppermost surface of theconductive base layer, the first depth being less than the thickness ofthe conductive base layer; forming a spacer layer along verticalsidewalls of, and overlying a bottom surface of, the recess; removing aportion of the spacer layer overlaying the bottom surface of the recessthrough a first etch process to expose a portion of the conductive baselayer; removing the portion of the conductive base layer through asecond and a third etch process, which forms an opening in theconductive base layer and exposes a portion of the dielectric layer;removing the portion of the dielectric layer through a fourth etchprocess, which extends the opening and exposes a portion of the uppersurface of the semiconductor substrate, wherein the extended opening hasan outer perimeter corresponding to inner sidewalls of the conductivebase layer; and selectively disposing silicon-germanium (SiGe) along theportion of the upper surface of the semiconductor substrate, and alongthe inner sidewalls of the conductive base layer, wherein the SiGe formsa SiGe base region within the conductive base layer, the SiGe baseregion having the second conductivity type; forming an emitter structureover the SiGe base region, wherein the emitter structure has outersidewalls which are more widely spaced than the inner sidewalls of theconductive base layer.
 2. The method of claim 1, wherein the first etchprocess is an anisotropic etch process.
 3. The method of claim 1,wherein the second etch process is an anisotropic selective etch with anetch selectivity between the conductive base layer and the dielectriclayer, such that it etches the portions of the conductive base layerwhile leaving the dielectric layer substantially intact.
 4. The methodof claim 1, wherein the third etch process is an isotropic selectiveetch with an etch selectivity between the dielectric layer and theconductive base layer, such that the third etch process laterally etchesthe portions of the conductive base layer.
 5. The method of claim 1,wherein the fourth etch process is a selective etch with an etchselectivity between the dielectric layer and the conductive base layer,such that it etches the portions of the dielectric layer while leavingthe conductive base layer substantially intact.
 6. The method of claim1, wherein the selectively disposing of the SiGe to form the SiGe baseregion comprises epitaxially growing the SiGe so a concentration of Gein the SiGe base region is graded to make a bandgap of the SiGe baseregion narrower at a first junction where the collector region meets theSiGe base region than at a second junction where the SiGe base regionmeets the emitter structure.
 7. A method, comprising: forming acollector region within a semiconductor substrate, the collector regionhaving a first conductivity type; forming a first dielectric layer overan upper surface of the semiconductor substrate; forming a conductivebase layer over the first dielectric layer, the conductive base layerhaving a second conductivity type that is different from the firstconductivity type; forming a first recess within the conductive baselayer, wherein the first recess terminates within the conductive baselayer and does not extend into or reach the first dielectric layer, andhas a pair of sidewalls which are spaced laterally apart by a firstdistance; forming a pair of spacers along the pair of sidewalls of thefirst recess; performing one or more etches with the pair of spacers inplace to form a second recess that extends downwardly through the firstdielectric layer to expose the upper surface of the semiconductorsubstrate, the second recess undercutting lower surfaces of the pair ofspacers and having sidewalls that are spaced apart by a second distancethat is greater than the first distance; forming a base region in thesecond recess, the base region having the second conductivity type;forming an emitter layer in the first recess and over the base region,the emitter layer having the first conductivity type; forming a maskover the emitter layer, the mask including inner sidewalls correspondingto an opening in the mask and which are spaced apart by a third distancethat is greater than the second distance; and performing an etch withthe mask in place to etch through the emitter layer and leave an emitterstructure whose outer sidewalls are spaced apart by substantially thethird distance and by greater than the second distance.
 8. The method ofclaim 7, wherein the base region is formed to be in direct contact withthe collector region.
 9. The method of claim 8, wherein the firstdielectric layer separates the conductive base layer from thesemiconductor substrate.
 10. The method of claim 8, wherein the baseregion is formed in the second recess to be in direct contact with asidewall or lower surface of the conductive base layer.
 11. The methodof claim 10, further comprising: forming a second dielectric layer overthe emitter layer; and forming a base contact through the seconddielectric layer to contact the upper surface of the conductive baselayer.
 12. The method of claim 11, further comprising: forming acollector contact through the second dielectric layer to contact anupper surface of the collector region.
 13. The method of claim 7,wherein the emitter layer is formed to be in direct contact with thebase region.
 14. The method of claim 7, wherein the base region is asilicon-germanium (SiGe) base region.
 15. The method of claim 14,wherein the SiGe base region is selectively deposited in the secondrecess without being formed along inner sidewalls of the pair ofspacers.
 16. The method of claim 7, wherein the base region is a silicongermanium (SiGe) base region, and wherein forming the SiGe base regioncomprises epitaxially growing the SiGe base region so a concentration ofGe in the SiGe base region is graded to make a bandgap of the SiGe baseregion narrower at a first junction where the collector region meets theSiGe base region and wider at a second junction where the SiGe baseregion meets the emitter.
 17. A method, comprising: forming a collectorregion within a semiconductor substrate, the collector region having afirst conductivity type; forming a first dielectric layer over an uppersurface of the semiconductor substrate and over the collector region;forming a conductive base layer over the dielectric layer, theconductive base layer having a second conductivity type that isdifferent from the first conductivity type; forming a second dielectriclayer over the conductive base layer; forming a first recess extendingthrough the second dielectric layer and extending to a depth into theconductive base layer, wherein the first recess terminates at a firstdepth from an upper surface of the conductive base layer which is lessthan a second depth from the upper surface of the conductive base layerto a lower surface of the conductive base layer, and has a pair ofsidewalls which are spaced laterally apart by a first distance; forminga pair of spacers along the pair of sidewalls of the first recess, thepair of spacers having outer sidewalls contacting the conductive baselayer and the second dielectric layer; and performing one or more etcheswith the pair of spacers in place to form a second recess that extendsdownwardly through the first dielectric layer to expose the uppersurface of the semiconductor substrate, the second recess undercuttinglower surfaces of the pair of spacers and having sidewalls that arespaced apart by a second distance that is greater than the firstdistance; forming a base region in the second recess, the base regionhaving the second conductivity type and not filling the first recess;forming an emitter layer in the first recess and over the seconddielectric layer, the emitter layer having the first conductivity type;forming a mask over the emitter layer, the mask including innersidewalls corresponding to an opening in the mask and which are spacedapart by a third distance that is greater than the second distance; andperforming an etch with the mask in place to etch through the emitterlayer and the second dielectric layer, thereby leaving an emitterstructure whose outer sidewalls are spaced apart by substantially thethird distance and by greater than the second distance.
 18. The methodof claim 17, wherein the base region is formed in the second recess tobe in direct contact with a sidewall or lower surface of the conductivebase layer.
 19. The method of claim 18, wherein the emitter layer isformed to be in direct contact with the base region.
 20. The method ofclaim 17, wherein the base region is a silicon-germanium (SiGe) baseregion.